Fuel cells are used to produce electricity when supplied with fuels containing hydrogen and an oxidant such as air. A typical fuel cell includes an ion conductive electrolyte layer sandwiched between a cathode layer and an anode layer. There are several different types of fuel cells known in the art, one of which is a solid oxide fuel cell (SOFC). A SOFC is regarded as a highly efficient electrical power generator that produces high power density with fuel flexibility.
In a typical SOFC, air is passed over the surface of the cathode layer and a reformate hydrocarbon fuel is passed over the surface of the anode layer opposite that of the cathode layer. Oxygen ions from the air migrate from the cathode layer through the dense electrolyte to the anode layer in which the oxygen ions reacts with the hydrogen and carbon monoxide in the fuel, forming water and carbon dioxide; thereby, creating an electrical potential between the anode layer and the cathode layer. The electrical potential between the anode layer and the cathode layer is typically about 1 volt and power around 1 W/cm2. Multiple SOFCs are stacked in series to form a SOFC stack having sufficient power output for commercial applications.
The anode acts as a catalyst for the oxidation of hydrocarbon fuels and has sufficient porosity to allow the transportation of the fuel to and the products of fuel oxidation away from the anode/electrolyte interface, where the fuel oxidation reaction takes place. The anode of a typical SOFC is typically formed of a nickel/yttria-stabilized zirconia (Ni/YSZ) composition. The use of nickel in the anode is desirable for its abilities to be a catalyst for fuel oxidation and current conductor.
Although nickel is a desirable hydrogen oxidation catalyst, nickel also catalyzes the formation of carbon from hydrocarbons under reducing conditions. Over time, the carbons atoms are deposited onto the surface of the Ni/YSZ based anode. As the number of carbon atoms deposited on the surface of the anode increases, the level of damage and deactivation of the anode from carbon formation increases dramatically. Also, prolonged steady state operation at elevated temperatures, which is between typically between 600° C. to 900° C. for a SOFC stack, causes the nickel in the Ni/YSZ composition to coarsen due to grain growth. The coarsening of the granular microstructure of the anode further reduces the efficiency of the anode for fuel oxidation. Furthermore, the Ni/YSZ anode is susceptible to contaminates, such as sulfur, in the fuel stream; sulfur compounds are known to poison the Ni/YSZ based anodes, thereby deactivating the SOFC stack.
There is a long felt need for a SOFC stack that has anodes that are minimally susceptible to degradation due to carbon deposits, Ni grain growth, and sulfur poisoning. There is also a long felt need to be able to treat the Ni/YSZ anodes of an existing SOFC stack in situ to reduce the susceptibility to carbon deposits and Ni/YSZ substrate grain growth.